A Stormont department is no longer defending a legal challenge to Northern Ireland's biggest roads project, the High Court heard today.

Senior counsel for the Department for Infrastructure asked a judge to quash its decision to press ahead with the A5 Western Transport Corridor scheme.

The concession is based on approval for the multi-million pound dual carriageway upgrade having been given in the absence of a minister.

Tony McGleenan QC said: "The Department considers it no longer in the public interest to defend these proceedings."

Lawyers representing a campaign group behind the legal action were given time to consult with their clients before a final order is made.

Adjourning the hearing for those discussions to take place, Mr Justice McCloskey suggested it was inconceivable they could secure any better outcome.

"This is as good as it can possibly get," he said.

The roads project linking Counties Derry and Tyrone has been hit by delays due to court challenges.

It would involve a new 85km trunk road running from New Buildings, via Strabane, Newtownstewart, Omagh and Ballygawley, and terminating near the border at Aughnacloy.

Construction on phase one was due to begin early this year at a cost of £150m.

But that was put on hold after fresh judicial review proceedings were launched by umbrella group the Alternative A5 Alliance.

In 2013 the group, made up of landowners, farmers and supporters, won its first legal action against the project.

At that stage a judge quashed the decision to press ahead with the scheme, which forms part of a proposed key cross-border business route linking Dublin and the north west, due to a breach of a habitats directive.

In November last year the Department announced its decision to proceed with the planned route - prompting the renewed challenge.

Lawyers for the Alliance were set to contest the legality of the decision being taken by the Department's Permament Secretary in the absence of a Minister.

Their position was strengthened by a landmark ruling earlier this year in a case over a planned incinerator near Belfast.

The Court of Appeal held that a senior civil servant did not have the legal power to take such decisions without a Minister being in post.

Even though new legislation has been brought in to give greater clarity, issues still surround existing challenges.

Informing the judge of developments in the Alternative A5 Alliance litigation, Mr McGleenan said: "The position is that the Department are inviting the court to quash the impugned decisions in this case."

He added: "It's hopefully clear that the issue at the heart of this is the absence of a minister when the original decision was taken."

A solicitor for the group behind the challenge requested time to consult with them.